No. 1.— New and old Silurian Trilobites from Southeastern Wisconsin, 
with Notes on the Genera of the Illaenidae. 
By Percy E. RaymMonp. 
In the F. H. Day collection, received in January 1881, as a gift of 
Mr. Alexander Agassiz, the Museum of Comparative Zodlogy secured 
one of the three great collections of the Silurian fossils of southeastern 
Wisconsin. It is particularly rich in Illaenidae, but contains also 
such rare forms as Harpes telleri, Trochurus nasutus, and Dicrano- 
peltis telleri, to be found elsewhere only in Mr. Teller’s magnificent 
collection. 
The trilobite fauna of the quarries around Milwaukee and Racine 
differs considerably from that found in the vicinity of Chicago, so 
that some of the species described by Weller from the latter area are 
either absent from the M. C. Z. collection or represented by speci- 
mens from other sources than the Day collection. The Phacopidae 
have been omitted from the present study, and only such species are 
mentioned as are represented by specimens which add something to 
what has already been published. 
ILLAENIDAE Hawle and Corda. 
The Illaenidae form a remarkably homogeneous group, and in spite 
of the great number of species which have been described only three 
generic or subgeneric names (Illaenus, Bumastus, and Thaleops) are 
in common use. Holm recognized only Illaenus and Bumastus and 
other writers have been even more conservative, referring all the 
species to Illaenus. The only serious attempt to subdivide the genus 
is that made by Salter, who recognized eight subgenera (including 
Illaenus), but none of Salter’s names has ever come into general use, 
although some of them could be adopted advantageously. In all, 
seventeen subgenera or genera have been proposed for inclusion in 
this family, but only seven of these seem to be valid. The names, in 
chronological. order, are:—Cryptonymus Eichwald 1825, Illaenus 
Dalman 1826, Deucalion Shtsheglov 1827, Bumastus Murchison 
1839, Archegonus and Dysplanus Burmeister 1843, Thaleops Conrad 
1843, Alceste Hawle and Corda 1847, Rhodope Angelin 1854, Actino- 
